Apparatus for measuring a state variable in a gas with at least one semiconductive gas sensor

Abstract

To eliminate the effects of semiconductive sensor aging in the use of a semiconductive sensor for measuring the state variables of a gas in a system in which a pair of electrodes are embedded in an active semiconductive layer so that the conductivity of the semiconductive layer varies as a function of the state variable, a field electrode is provided and superimposed upon the measurement voltage, a voltage pulse is applied intermittently and the characteristic of the resulting current pulse is used to compensate for changes resulting from aging of the sensor.

We claim: 
     
       1. An apparatus for measuring a state variable in a gas, comprising: a semiconductive gas sensor disposed in a gas having a state variable to be monitored, said semiconductive gas sensor comprising:   an insulating layer,   a field electrode on said insulating layer,   an active semiconductor layer on said insulating layer and spaced by said insulating layer from said field electrode, and   two spaced apart further electrodes embedded in said semiconductor layer and isolated from said field electrode;   a power supply connected in circuit with said further electrodes;   a measurement-signal processor connected in circuit with said power supply and said further electrodes of said semiconductive gas sensor for detecting a signal across said semiconductive gas sensor representing conductivity of said semiconductive layer and said state variable;   a voltage-pulse generator connected across said field electrode and at least one of said further electrodes for applying time-spaced measurement pulses between said field electrode and said one of said further electrodes for autogenous control of sensor functioning by generating conductivity changes in said semiconductive layer as a function of time which are measured by said measurement-signal processor for each of said pulses; and   means responsive to the measurement pulses for superimposing upon each of said measurement pulses a respective second measurement signal which is modified by aging of the semiconductive layer.   
     
     
       2. The apparatus defined in claim 1 wherein said semiconductive layer and said further electrodes are disposed one side of said insulating layer, and said field electrode is disposed on an opposite side of said insulating layer. 
     
     
       3. The apparatus defined in claim 1 wherein said field electrode, said semiconductive layer and said further electrodes are disposed on one side of said insulating layer, a covering glass insulating layer is applied to said field electrode to insulate said field electrode from said semiconductive layer. 
     
     
       4. The apparatus defined in claim 1, further comprising a conductor structure on said insulating layer connectable in a heating circuit for resistive heating of said sensor. 
     
     
       5. The apparatus defined in claim 1 wherein said insulating layer is a thin layer of a thickness of 30 to 100 nm. 
     
     
       6. The apparatus defined in claim 1 wherein said insulating layer is composed of silicon dioxide, silicon nitride, aluminum oxide or mixtures thereof. 
     
     
       7. The apparatus defined in claim 4, wherein said conductor structure is composed of a highly conductive metal or doped polysilicon. 
     
     
       8. The apparatus defined in claim 1 wherein said voltage pulse generator is constructed and arranged to generate a field strength of 10 4  to 10 6  V/cm. 
     
     
       9. The apparatus defined in claim 1 wherein said power supply is constructed and arranged to apply a voltage of about 10 V across said further electrodes. 
     
     
       10. The apparatus defined in claim 1, further comprising a microcontroller connected to said power supply and said generator for controlling progress of a measurement of a state variable and autogenous function control. 
     
     
       11. The apparatus defined in claim 1 wherein, for operation at a critical measuring range of the active semiconductive sensor, said sensor has a heating structure thereon connectable to a heating circuit. 
     
     
       12. The apparatus defined in claim 1 wherein said sensor is operated with direct current or with periodically applied direct current. 
     
     
       13. The apparatus defined in claim 10, further comprising a conductor structure on said insulating layer connectable in a heating circuit for resistive heating of said sensor. 
     
     
       14. The apparatus defined in claim 13 wherein said insulating layer is a thin layer of a thickness of 30 to 100 nm. 
     
     
       15. The apparatus defined in claim 14 wherein said insulating layer is composed of silicon dioxide, silicon nitride, aluminum oxide or mixtures thereof. 
     
     
       16. The apparatus defined in claim 15, wherein said conductor structure is composed of a highly conductive metal or doped polysilicon. 
     
     
       17. The apparatus defined in claim 1 wherein said voltage pulse generator is constructed and arranged to generate a field strength of 10 4  to 10 6  V/cm and said power supply is constructed and arranged to apply a voltage of about 10 V across said further electrodes. 
     
     
       18. The apparatus defined in claim 17 wherein said semiconductive layer and said further electrodes are disposed on one side of said insulating layer, and said field electrode is disposed on an opposite side of said insulating layer. 
     
     
       19. The apparatus defined in claim 18 wherein said field electrode, said semiconductive layer and said further electrodes are disposed on one side of said insulating layer, a covering glass insulating layer is applied to said field electrode to insulate said field electrode from said semiconductive layer. 
     
     
       20. In a method of measuring a state variable in a gas with an apparatus which comprises: a semiconductive gas sensor disposed in a gas having a state variable to be monitored, said semiconductive gas sensor comprising:   an insulating layer,   a field electrode on said insulating layer,   an active semiconductor layer on said insulating layer and spaced by said insulating layer from said field electrode, and   two spaced apart further electrodes embedded in said semiconductor layer and isolated from said field electrode;   a power supply connected in circuit with said further electrodes;   a measurement-signal processor connected in circuit with said power supply and said further electrodes of said semiconductive gas sensor for detecting a signal across said semiconductive gas sensor representing conductivity of said semiconductive layer and said state variable;   a voltage-pulse generator connected across said field electrode and at least one of said further electrodes for applying time-spaced measurement pulses between said field electrode and said one of said further electrodes for autogenous control of sensor functioning by generating conductivity changes in said semiconductive layer as a function of time which are measured by said measurement-signal processor, the improvement which comprises the steps of:   generating for each of said measurement pulses a respective second measurement signal,   superimposing each second measurement signal upon the respective measurement pulse, each second measurement signal being modifiable in response to aging of the semiconductive layer, and   evaluating a change in a rise time of said second measurement signal for each of said measurement pulses as a function of aging.